1. Field of the Invention
This invention relates to a sensor unit used for a detector for a fuel gauge, a hydraulic gauge or the like of a vehicle, and more particularly to an improvement in a sensor unit for detecting a variation of an object to be detected such as a liquid level, hydraulic pressure or the like in the form of an electrical signal.
2. Description of the Prior Art
Conventionally, various kinds of sensor units have been proposed which are adapted to detect liquid pressure such as hydraulic pressure of a gasoline engine, liquid level of fuel in a fuel tank or the like. Such conventional sensor units typically include a signal converting means for converting the amount of movement of a drive arm shifted depending on a variation of an object to be detected (hereinafter referred to as "detected object") such as a liquid level, liquid pressure or the like into an electrical signal. A signal generated from the signal converting means is externally fed through a connection conductor for connecting an output electrode of the signal converting means and an output terminal provided on a protection cover with each other. Conventionally, as such a signal converting means is generally used a variable resistor which is so constructed that a linear resistor element is wound two or more times at predetermined intervals on an insulating substrate to form a wire wound resistor element and a contact of a slider is slid on the wire wound resistor element. However, it is known in the art that the variable resistor of this type exhibits various disadvantages.
In view of the disadvantages, the assignee proposed to use a variable resistor which includes a printed resistor board formed by depositing a resistor pattern and an output electrode on an insulating substrate by printing. Unfortunately, it was found that the variable resistor proposed has the following defects.
One of the defects is encountered with mounting of the substrate on a fixing bracket. In the above-described conventional variable resistor which uses the wire wound resistor, it is possible to fix the insulating substrate on a fixing bracket by riveting because the substrate may be formed in a manner to exhibit relatively high mechanical strength. On the contrary, in the variable resistor proposed by the assignee, a material for the insulating substrate is limited to that relatively low in mechanical strength such as ceramic or the like because it is required for the substrate to exhibit satisfactory printing characteristics. Accordingly, fixing of the insulating substrate on the fixing bracket by riveting tends to cause damage or breakage of the substrate. In order to avoid such a problem, those skilled in the art would take an approach of fixing the substrate on the bracket by means of adhesive. However, when a sensor unit is subjected to a high temperature and vibration during operation like a sensor unit for detecting hydraulic pressure of an engine of a vehicle, the adhesive is deteriorated to a degree sufficient to cause the substrate to be separated from the bracket. Also, use of adhesive requires much time for drying it and needs use of solvent, resulting in workability being deteriorated.
Another defect is that such a sensor unit is smallsized to a degree to fail to provide a sufficient interval between the output electrode and the resistor pattern, accordingly, it is required to carry out soldering between the output electrode and the connection conductor while taking much care to avoid adhesion of solder to the resistor pattern. Also, soldering flux often adheres onto a plurality of contact electrodes on the resistor pattern with which a contact of a slider is contacted. Such adhesion of the soldering flux leads to a variation of resistance of the resistor pattern.